Not Applicable
Not Applicable
This invention relates to a chiller plant, and specifically methods of operation that will reduce the power usage and improve the plants efficiency.
A chiller plant as shown in FIG. 1, includes a chiller (5), a cooling fluid pump (1), a cooling tower (3), and a cooling fluid piping circuit (11) which interconnects these pieces of equipment. The chiller plant produces a chilled fluid that is distributed in a chilled fluid piping circuit (12).
The chiller plant may also include a plurality of chillers, pumps, and cooling towers. For instance multiple chillers can be operated in parallel, with a single cooling fluid piping circuit connecting all chillers to the same cooling tower. Another system shown in FIG. 2 has a separate fluid cooling circuit, and separate cooling tower for each individual chiller in the chiller plant.
The chiller (5) is a refrigeration machine that chills water or other fluid mediums to a controlled temperature level. It is a complete unit consisting of a refrigerant condenser (4), a refrigerant evaporator (10), a refrigerant compressor (9), a chiller control panel (8), and an electric motor drive (7). The chiller (5) is typically provided as a single package by one manufacturer. The cooling tower (3), cooling fluid pump (1), and cooling fluid piping circuit (11) constitutes a heat rejection system for the chiller plant. The cooling tower (3) can have one fan (6) or a plurality of fans. The cooling tower (3) fans (6) and the cooling fluid pump (1) have electric motor drives (7). The electric motor drives (7) may be constant speed or variable speed.
The Cooling fluid pump (1) forces a cooling fluid (2) to circulate from the cooling tower (3) to the refrigerant condenser (4) of the chiller (5) and back to the cooling tower (3). When chiller (5) is operating pump (1) operates at a constant flow rate. Fan (6) for cooling tower (5) also operates independently of the chiller.
The chiller, the cooling tower fan, and the cooling fluid pump all have control systems that are independent of each other. During periods of operation, when the chiller is operating at part load, the cooling tower fan, and the cooling fluid pump will be operating at full or a high and unnecessary power setting, thus wasting a substantial amount of energy. Since the chiller typically operates at part load for most of the time, the amount of wasted energy over time can be quite large.
The chilled fluid temperature is controlled by the chiller to a set temperature. The chiller set point can be adjusted by the chiller plant operator, either manually, or through an automatic control system provided for the plant. When loading levels on the plant are low, it can be desirable to reset the chilled fluid temperature to a higher level to reduce energy usage.
Various control systems have been applied in the past, which have automated portions of the chiller plant and it""s heat rejection system. The automation was initially designed to eliminate manual control, then additional automation for better control and to save energy, which quickly becomes obsoleted with the availability of better instrumentation and computerized controllers.
FIG. 2 shows a chiller plant with a plurality of chillers, pumps and cooling towers. This depicts a method for controlling cooling fluid pumps, cooling tower fans, and chiller operations in a coordinated fashion to reduce energy usage.
A computerized controller (13) controls the speed of the electric motor drives (7) for the cooling tower fan (6), the cooling fluid pump (1), and chiller (5).
U.S. Pat. No. 6,257,007 to Hartman (2001), and U.S. Pat. No. 6,185,946 to Hartman (2001) describe methods similar to FIG. 2, where the system components of the chiller plant are controlled in response to the current loading level on the cooling system. The current loading level is always determined by specific chiller parameters such as power, or refrigerant head pressure, or motor speed. Therefore, these methods may require direct access to the selected chiller parameters. Since the chiller manufacturer normally does not provide for this type of access, it can only be implemented with the special help from the manufacturer.
U.S. Pat. No. 5,963,458 to Cascia (1999) describes a generic computer designed to use chiller load data derived from the chiller plus additional parameters that include wet bulb temperature, tower air flow rate, and condenser water flow rate. The computer then determines the optimal set point for operation of cooling tower fans, condenser water pumps, and chillers. The computer can be set up to provide set point operation to as many or as few components as desired. The electric motor drives can be either variable speed or constant speed. A large number of peripheral devices are required and the control sequence can only be implemented through this generic computer with a set of complicated and difficult to understand control algorithms. It can only be implemented with the help of highly trained specialists.
Another method for the control of the cooling fluid pump is disclosed in U.S. Pat. No. 5,070,704 to Conroy. A plurality of chillers are served by one cooling fluid pump. The chillers each have control valves which shut off cooling fluid flow when that chiller is off line. The cooling fluid pump has a variable speed electric motor which is controlled by way of a pressure sensor located in the cooling fluid circuit. This control has limited application in today""s chiller plant that typically has separate pumps matched to each chiller.
A number of methods have been disclosed for the control of cooling tower fans. U.S. Pat. No. 4,085,594 to Mayer, (1978) controls fans in response to the temperature of the cooling fluid, as is also shown in and U.S. Pat. No. 4,252,751 to Shito, (1981). The principle intention was to automate the cooling tower control and eliminate man power. More efficient controls were introduced with U.S. Pat. No. 4,474,027 to Kaya, et al, (1984), which discloses the use of wet bulb temperature to optimize the speed of the cooling tower fans. Additional improvements in energy usage are introduced in U.S. Pat. No. 5,040,377 to Braun et al, (1991) and U.S. Pat. No. 5,600,960 to Schwedler et al, (1997), both include a means to determine the chiller load by measuring the temperature and flow rate of chilled fluid that enters and exits the evaporator, as well as a means for determining wet bulb temperature and cooling fluid temperature, then control the cooling tower fan to the desired speed. Multiple input parameters are required, which must be compared using multiple logic loops to finally determine the desired control output, making a complicated control regime, requiring highly trained specialists to implement.
Several objects and advantages of the present invention are:
(a) to provide method to maintain a constant, or near constant, temperature difference of the cooling fluid as it enters and exits the refrigerant condenser.
(b) to provide a method to control the flow rate of the cooling fluid to the refrigerant condenser.
(c) to provide a simple method to control the cooling tower fan speed based on the cooling fluid temperatures and cooling fluid flow rate.
(d) to provide a simple method to allow sequencing of a plurality chillers, and optimize combined energy efficiency of the operating chillers.
(e) to provide a method of control for condenser water pumps, and cooling tower fans of a chiller plant that has a constant speed chillers and variable speed chillers in the same plant.
(f) to provide a method of control that is simple to install and maintain.
(g) to provide a method of control that is flexible and adaptable to different chiller plant designs.
(h) to provide a method of control that does not require direct access to proprietary wiring and controls of a manufacturer""s chiller.
(i) to provide a method of control that can be retrofitted to existing chiller plants.
Further objects and advantages are to provide a simple method of control that uses off the shelf components, can be provided as a stand-alone system without the support of a building automation system, can also be incorporated into major building automation systems, is flexible in the inclusion or exclusion of controlled components, does not require proprietary knowledge of chiller manufacturer""s equipment, can be easily employed by the engineering and construction disciplines. Still further objects and advantages will become evident from a consideration of the ensuing description and drawings.
In accordance with the present invention it provides a methodology for the control of chiller plants that will improve the combined system efficiency of the various pieces of equipment to improve the overall energy usage.